By using CMOS processes at present it is possible to provided in a single integrated circuit complex systems comprising both analog and digital parts.
For the analog parts, provision of valid operational transconductance amplifiers (OTA) is essential.
Indeed, these amplifiers are the basis for the provision of many complex circuits such as filters or other circuits with switched, coding, decoding, etc., capacitors, i.e., condensers. These amplifiers fall in the broader class of operational amplifiers but are characterised by rather high output impedance even with closed loop. For this reason these amplifiers are particularly well suited in structures with feedback and capacitive load such as for example the above mentioned switched condenser circuits. But for resistive feedback, because of the high output impedance, even higher feedback resistors should be used and hence their use is not always possible.
There has been a considerable ongoing effort to improve the performance of these amplifiers. On one hand it is sought to improve their gain-band product, output dynamics, supply disturbance rejection noise and output piloting. On the other hand, it is sought to reduce the supply voltages to make them equivalent to those of digital circuits which, ever more often, operate typically at 3.3 V with minimum of even 2.7 V for battery supply.
The first solutions used by those skilled in the art were rather simple structures but did not offer high performance, especially in terms of stability.
A known OTA circuit topology represented in FIG. 1 ("Design techniques for cascode CMOS op amp with improved PSRR and common mode input range", Ribner--Copeland JSSC 12/84) includes a first cascoded stage and a second gain stage. A compensation capacitance, instead of being connected as in older solutions between the OTA output and the first stage output is connected to the source terminal of an M5 transistor with grounded gate terminal acting as the cascode. This simple variation considerably improved the circuit. In this manner, a zero to the right resulting from the frequency analysis is taken to a slightly higher frequency (as shown in the above mentioned article) and hence degradation of the phase margin introduced by it is less. There is also another zero to the left which further helps improve amplifier stability.
Compared with previous transconductance amplifiers, a predominant pole remains unchanged. However, instead of a second pole, there is a doublet of conjugate complex poles shifted to a slightly higher frequency. The shift factor is CC/C1 where CC is the compensation capacitance and C1 is the parasite capacitance at the first stage output.